A desire to transmit more data in less time has resulted in an increased interest in using optical fiber to transmit data instead of copper wires. Optical fiber can carry more data than a similarly sized electrical cable and is less susceptible to electrical and magnetic interference. While optical fiber is commonly used for long distance telephone and Internet backbone communications, it has not found significant use over shorter distances.
For many shorter distance optical connections, cost becomes a more significant factor. With reductions in the cost of the transmitting and receiving equipment on both sides of an optical fiber, jitter becomes more difficult to manage. Jitter is a variation in the phase, or timing of a series of signals that is caused by interference, cross-talk, inter-symbol interference, limitations in transmitters and receivers and other aspects of the connection. While jitter can be present in almost any data connection whether electrical, optical, or radio, as the frequency of the data increases, jitter becomes more important. Jitter is used to describe a wide variety of different effects on a data stream. Many of these relate to changes in phase or timing.
In an optical fiber transmission system, the data is driven by an electrical transmitter which modulates the incoming data stream which creates an input to the optical transmitter. The optical transmitter converts the electrical signal into an optical signal which is carried by the optical fiber and is typically recovered by a photo detector. The photo detector generates an electrical signal that is high or low depending on whether the optical signal is on or off. This electrical signal is then carried to an electrical receiver and sampled at some frequency that corresponds to the timing of the transmitted data. Without jitter, if the timing of the received signal and the sampler match, then the sampler will observe either a high or a low. These highs and low correspond to zeroes and ones of the digital signal. If there is jitter in the signal then the cycles of the received signal may not always align with the timing of the sampler and the sampler may recover the wrong value.
In a typical optical transmission system, the optical fiber is coupled at the receive end to an optical module with a photo detector. In many cases, the optical module includes a limiter to limit the total amplitude of the converted electrical signal from the photo detector and related amplifiers. The signal at the limiter's output typically has high jitter most of which is Data Dependent Jitter (DDJ) caused in part by Inter-Symbol Interference in the optical fiber.